PCC is precipitated calcium carbonate. In industry it is typically produced by allowing calcium hydroxide Ca(OH)2 to react with a precipitating agent, such as, for example, carbon dioxide CO2, wherein the resulting product is precipitated calcium carbonate (PCC). The calcium hydroxide Ca(OH)2 used in the reaction may be produced, for example, by slaking burnt lime CaO with water, wherein the reaction produces calcium hydroxide.
Typically calcium carbonate CaCO3 is precipitated in industry by introducing gas containing carbon dioxide into a suspension of calcium hydroxide in water, which is also known as milk of lime. Calcium carbonate precipitated in such a manner may be used, for example, as paper filler or coating pigment. By changing process conditions it is possible to influence the crystalline and granular form of precipitated calcium carbonate and its properties.
The use of precipitated calcium carbonate in paper improves the optical properties and the printing properties of paper, and in addition, it typically reduces production costs per manufactured ton of paper. U.S. Pat. No. 5,679,220, U.S. Pat. No. 5,731,080 and EP-1297220 disclose methods of prior art for precipitating calcium carbonate in the fibre raw material of paper.
Increasing the calcium carbonate content of paper or paperboard typically results in cost savings, which are mainly due to the decreasing percentage of the amount of more expensive fibre material. Therefore, it is generally an aim in paper manufacture to introduce as much filler in paper as possible. Increasing filler content according to prior art, however, typically causes deterioration in the strength properties of paper. Especially decreasing the wet strength of paper may cause significant problems in a paper machine. Therefore, in industry there is a need for a solution that would facilitate increasing the filler content of a product being produced while not decreasing the wet strength as significantly as in solutions of prior art.